Ultracold Atoms

In the quest to reach colder and colder temperatures, physicists in 1995 created a remarkable new form of matter—neither gas, nor liquid, nor solid—called a Bose-Einstein condensate (BEC). First envisioned by Albert Einstein and a young Indian physicist named Satyendra Bose in the 1920s, BECs reveal properties of quantum mechanics—their atoms seem to merge together and lose their individual identity, behaving less like discrete particles and more like waves. If you're having trouble picturing this, not to worry; even physicists who work with BECs find them mind-boggling. In this interview, Luis Orozco of the University of Maryland, College Park offers some metaphors to help us begin to comprehend BECs and get a grasp of research in the strange world of ultracold atoms.

Suspended animation

NOVA: What happens to the world when you get down to really low temperatures, and why is that of interest to you?

Luis Orozco: Probably the most important thing that low temperature brings us is that things move slowly, and as things start to move slower and slower you're able to look at them for extended periods of time. It is as if I were to ask you, "Could you tell me something about the handles of a car that is passing on a highway at 50 or 60 miles an hour?" Definitely you won't be able to say anything. But if the car is moving rather slowly, then you would be able to tell me, "Oh yes, the handle is this kind, that color, has these properties." Or: "There is no handle." And it's precisely that ability to interrogate the car—I am looking at the car for a long, long time—so I can get information out of it.

In the same sense, if we have cold atoms and they're moving very, very slowly, then I should be able to learn a lot and get information out of those atoms. I extend the time that they are available for me. Now, if I slow them enormously and I trap them, that would even be better, because now I have not just one path, but the atom will come back and will come back and will come back as if it were looking for a parking space in downtown Manhattan. So that atom is trapped going around and around. Now, many times we don't want to just work with one atom, we may want to have more and more atoms, but again it would be much better to have them moving slowly.

At room temperature an atom is moving at roughly 500 meters per second, so those are quite a few miles an hour [about 1,100 mph]. However, if I slow it to a temperature that we can now achieve without much work in the lab, 200 microKelvin [about minus 460°F], then the atoms start to move about 20 centimeters per second [about 0.45 mph]. Compared to something that's rushing in front of you, you'd be able to look at a lot of the details, a lot of the internal structure of that atom.

Now, why are we, in general, interested in that? Probably the first thing is because we want to have a better clock, and it is from the interrogation of that atom that we get the definition of the second. And by further cooling the atoms we have better clocks, and better clocks are giving us better global positioning systems, which are now a part of our daily life.

"Some people talk about the effects of BECs in things like neutron stars, where there could be really large condensates."

What do we want to find out about atoms that we can't find out unless they're supercold?

There are many things that we don't know about atoms, and one of those things in particular that I am interested in is the weak force. Imagine that you have to explain the weak force to your grandmother. Your grandmother remembers her high school chemistry, which says that the sun burns hydrogen and produces helium. But your grandma is very bright and says, "Yes, but if the sun only has hydrogen, how do you get helium out of that? Where did the neutron come from, because hydrogen doesn't have a neutron and helium has two?" And you say, "Hey, there is a weak force—that's how the sun starts its cycle, that's the key to how to convert a proton into a neutron. The weak force is, as its name says, very, very weak, and it has very few effects, but the most important effect is a striking one."

The weak force was predicted and found exactly 50 years ago, but we still have a lot to learn from it. It's a very, very small detail on the handle of the car, so we need to look at it for a very long time. We need to interrogate it very, very delicately. It is analogous to trying to measure the size of the Earth in its diameter and then changing it by a hair and then again finding what the diameter is. That's the rough scale.

So when we started this project to study the weak force, we definitely needed very, very slow atoms, and we not only needed slow atoms, we needed to confine them very carefully, put them in a trap so that we could interrogate, we could ask over and over and over the same question to eventually learn something about the weak interaction.

A quantum leap

In addition to slowing things down, what else can happen to atoms when you are in the realm of the ultracold?

There is a second fascinating part that we haven't talked about and that is coherence. Coherence is that wonderful property that means things are correlated. A very good example is a soccer game. When you are in a soccer stadium, everybody is shouting all the time, and the noise level is pretty high. Then, what happens when there is a scoring goal? Every single person shouts "Goal!" at the same time, and there is this incredibly loud sound. Nobody is shouting louder than they were before, but they are shouting coherently. So by having the atoms ultracold, I can achieve much better coherence. When I ask the atoms in the right way, they will shout "Goal!" back to me in a very loud voice—that's the Bose-Einstein condensate (BEC).

How many atoms can you get together with such coherence? In other words, is there a limit to how big a BEC can be?

Well, I don't think there is a [theoretical] limit on how big a BEC can be. What limits us is more a matter of the "stadium" [the apparatus or environment in which the BEC is created]. Some people talk about the effects of BECs in things like neutron stars, where there could be really large condensates, and of course we get some of their "Goal!" shouts by listening to pulsars and things like that. But in general, if you ask me right now what are the sizes that one can expect in the next few years for a BEC in a lab, I don't think we're going to get to a BEC of a gram; that's just too much.

What's limiting the size of the stadium that physicists can build?

There are various complications. The first complication, as you said, is really the size. As we make the stadium larger and larger, you have to control better and better the smoothness that you have in that environment. What is that smoothness? Well, that smoothness is the magnetic field. The environment has to be very, very well controlled.

There are other problems that can also happen. As I put more and more atoms in that stadium and I'm trying to cool them down, I'm trying to have them not move. I'm trying to also make them work coherently, but there are collisions. The individual people in the stadium are colliding with each other, and some are going to get upset and are going to leave. So that starts to happen at some point, and technically those bad collisions can limit you.

"We have to go to the ultracold, learn in the ultracold, and then bring that knowledge back into the room-temperature world."

Are there other problems with working with a large stadium, to use your analogy?

Imagine that you have an enormous stadium and the signal, the place where the goal is scored, is very far away. If you're a kilometer away, you're not going to see when they score. And so there is going to be a delay in your response, which is not good because that dilutes the coherence—the coherence is everybody jumping at the same time. So the speed of the propagation of the information across the space may be a problem.

I am sure that we are going to find ways to manage this. In many stadiums now they have big TV screens showing you what's going on. So probably we'll figure out a way to relay that information much faster to the other atoms, and then we'll be able to get larger and larger and larger condensates.

Potential applications

Given how difficult it appears to be to create a BEC, why are you so interested in it?

Well, one of the dreams that is starting to become a reality is to use condensates in the context of quantum information. There are lots of ideas, lots of proposals, though nothing yet terribly concrete, where you would say, "Oh look, here is your quantum computer." However, we know it's a good idea because of coherence. The coherence is there, the atoms will work in a coherent way, and that's much, much better than each one individually doing it. So that's an important area that may have applications. I think it's worth pursuing, and many people are pursuing it.

Speaking of practicalities, it seems perverse that you have to get down to these really low temperatures to achieve superconductivity, but to use superconductors, you really want them at room temperature ideally, don't you?

We have had a lot of difficulty quantitatively understanding high-temperature superconductors [materials that conduct electricity with no resistance at temperatures above the boiling point of liquid nitrogen (77 K or minus 321°F)]. We would like to have better high-temperature superconductors for a range of applications, from transferring electricity in a city, to medical applications, to transportation. So how are we going to do that?

Well, one of the things that we're learning about our ultracold world is that BECs are very simple systems and very good to manipulate. What we need to do is use the ultracold, use a BEC, and put that BEC in a lattice that has the same structure that a high-temperature superconductor can have. How would we make those lattices? We'd make those lattices with light. We'd force the atoms into a structure that is similar to the structure that the atoms have in the high-temperature superconductor. Then we can move and deform that pattern in such a way that we can see which patterns are better for superconductivity. Then we will be able to modify high-temperature superconductors to get much better applications. So to a certain extent we have to go to the ultracold, learn in the ultracold, and then bring that knowledge back into the room-temperature world. That would be wonderful.

"The BEC gives me control. All the atoms are the same, and I can create a pattern that is very uniform and has no defect."

Going back to your car analogy, is it a bit like doing a 3-D computer design of a car that gives you the knowledge to physically build the thing? Are you trying to mimic that three-dimensional pattern that you've got in your BEC? Would you then try and actually build a similar pattern, a similar lattice in a solid crystal or something along those lines?

Absolutely. The BEC gives me control. All the atoms are the same, and I can create a pattern that is very uniform and has no defect. No defect is very difficult to get in the real world. However, if I have that beautiful control, then I can find out what are the necessary elements in that pattern. Where do I have to put the money to create a better high-temperature superconductor? Do I have to put more effort in having this particular pattern just where the oxygen is in the superconductor? Or do I have to look at where the rare air is? And so on. That is a fantastic tool. In your computer 3-D simulation of the car, are there defects? That's when you can test it. You understand all the laws of aerodynamics, all the laws of physics, and everything is tested in your computer. That is the role that the BEC can have for these other areas.